A generator of high energy neutrons may be particularly useful for neutron-gamma ray or neutron-neutron logging in oil well logging tools, for example. One type of generator for use in a well logging tool, for example, is a sealed-tube neutron generator.
A sealed-tube neutron generator may have four major features. First, a sealed-tube neutron generator may have a gas source to supply reacting elements, such as, deuterium and tritium, for example. A sealed-tube neutron generator may also have an ion source that strips electrons from gas molecules, thus generating a plasma of electrons and positively charged ions. A sealed-tube neutron generator may also include a target impregnated with deuterium and/or tritium, and an accelerating gap which propels the ions from the plasma to the target with such energy that the bombarding ions collide and fuse with the deuterium or tritium nuclei of the target to generate and emit neutrons therefrom.
Ordinarily, a plasma of positively charged ions and electrons is produced by energetic collisions of electrons and uncharged gas molecules within the ion source. Two types of ion sources may be used in a neutron generator, for example, for well logging tools. One type is a cathode (a.k.a. Penning) ion source, and another type is a hot (a.k.a. thermionic) cathode ion source. These ion sources may include anode and cathode electrodes of different potential that contribute to plasma production by accelerating electrons to energy higher than the ionization potential of the gas. Collisions of those energetic electrons with gas molecules produce additional electrons and ions.
A Penning ion source may increase collision efficiency by increasing the distance that the electrons travel within the ion source before they are neutralized by striking a positive electrode. The electron path length is increased by establishing a magnetic field, which is perpendicular to the electric field within the ion source. The combined magnetic and electrical fields may cause the electrons to define a helical path within the ion source, which may substantially increase the distance traveled by the electrons within the ion source. Accordingly, the collision probability may be enhanced, and thus the ionization and dissociation efficiency of the device. Examples of a neutron generator that includes a Penning ion source used in a logging tool are described, for example, in U.S. Pat. Nos. 3,546,512 and 3,756,682 both to Fentrop, both of which are assigned the present assignee, Schlumberger Technology Corporation, and both of which are hereby incorporated by reference in their entirety.
A hot cathode ion source may include a dispenser cathode formed from a material that emits electrons when heated. A grid electrode extracts electrons from the cathode which, in turn, ionizes the gas, generating ions. An extracting or focusing electrode extracts ions and focuses such ions to form an ion beam. An example of a neutron generator that includes a hot cathode ion source used in a logging tool is described in U.S. Pat. No. 5,293,410 to Chen et al., assigned to the assignee of the present application, Schlumberger Technology Corporation, and the entire contents of which are hereby incorporated by reference.
In such a system, the target floats at a negative high voltage potential on the order of −70 kV to −160 kV (or less), for example, with the ion source electrodes operating around ground potential to provide the electric field gradient for accelerating ions toward the target with enough energy that the bombarding ions generate and emit neutrons therefrom. About 10 watts of power may be dissipated in the target, and the target is surrounded by high voltage insulation. Because of poor thermal conduction to the exterior (due to electrical insulators generally being poor thermal conductors), the temperature of the target can increase significantly compared to ambient temperature. At a relatively high ambient temperature, the target may overheat, which may thus lead to failure (loss of neutron output) of the neutron generator.